Engines having dual fuel systems are capable of consuming liquid fuel, such as gasoline, and gaseous fuel, such as propane. This dual fuel capability is advantageous when an engine is required to work indoors where the exhaust from a gasoline engine is unacceptable, and outdoors where the power requirements of the engine may exceed the capabilities of a propane burning engine.
Dual fuel systems for internal combustion engines, such as disclosed in U.S. Pat. No. 4,492,207, are known in the art. These systems typically require a venturi type mixer having a narrow control range for metering combustion air mixing with the gaseous fuel. The narrow control range of a venturi complicates the mixer design when the engine operation requires a high and low speed.
Furthermore, dual fuel systems are often integrated with the liquid fuel carburetor. A dual fuel carburetor is more expensive to make and has a limited market.